Liquid-crystal display device and process for producing liquid-crystal display device

ABSTRACT

The present invention provides a liquid-crystal display device capable of maintaining a high voltage holding ratio even without an alignment film. The liquid-crystal display device of the present invention includes a pair of substrates, a liquid crystal layer which is sandwiched between the substrates and which contains a liquid crystal material, and a polymer layer which is disposed on a surface of each of the substrates and which controls the alignment of liquid crystal molecules. The outermost surfaces of both the substrates comprise substantially no alignment film. The polymer layer is formed by polymerizing one or more radical polymerizable monomer species added to the liquid crystal layer. At least one species of the one or more radical polymerizable monomer species is a biphenyl compound having a specific structure.

TECHNICAL FIELD

The present invention relates to a liquid-crystal display device and aprocess for producing the liquid-crystal display device. The presentinvention specifically relates to a liquid-crystal display device whichis free from any known alignment film and which controls the alignmentof liquid crystal molecules using a polymer layer (hereinafter, alsoreferred to as a polymer sustained alignment (PSA) layer) that is formedby polymerizing a monomer contained in the liquid crystal composition,and a process for producing a liquid-crystal display device which issuitable for producing the polymer layer.

BACKGROUND ART

Since liquid-crystal display devices have a thin profile, a lightweight, and low power consumption, they are widely used as displayapparatuses such as televisions, personal computers, and PDAs. Recently,large-size liquid-crystal display devices have rapidly been introducedin various fields, such as liquid-crystal display devices fortelevisions. Such large-size devices suitably employ a multi-domainvertical alignment (MVA) mode because the MVA-mode display devices canbe produced at a high yield despite their large areas and can have awide viewing angle. With the multi-domain vertical alignment mode, theliquid crystal molecules are aligned orthogonally to the substratesurface when no voltage is applied to the inside of the liquid crystallayer. Thus, this mode can provide a higher contrast ratio than theconventional twisted nematic (TN) mode.

In the MVA mode, the tilting direction of the liquid crystal moleculesis controlled not by an alignment film but by the protrusions (ribs)formed of an insulating material. The alignment film thus requires noalignment treatment, eliminating static electricity and dust due to analignment treatment such as rubbing. This removes a requirement forwashing or the like treatment after the alignment treatment. Further,the liquid crystal molecules suffer less initial tilting variations.Therefore, this mode is effective to simplify the process, to increasethe yield, and to reduce the cost.

Although the MVA mode requires no alignment treatment, it still requiresformation of a base film which corresponds to an alignment film. Inorder to prevent the influences on the alignment of liquid crystalmolecules due to an uneven film thickness of this base film or due tocontamination by foreign material, and in order to prevent an increasein the number of steps or in the investment in plant and equipment forthe formation of a base film, it is more preferable not to form the basefilm itself.

In recent years, one technique has been focused on in which a liquidcrystal composition containing a liquid crystal material in admixturewith any polymerizable components such as monomers and oligomers(hereinafter, abbreviated as monomers) is injected into between thesubstrates, and then heat or light (e.g., ultraviolet radiation) isapplied to the composition to polymerize the monomers, thereby forming apolymer layer (PSA layer) (for example, see Patent Literature 1). Such apolymer layer makes it possible to maintain the alignment of liquidcrystal molecules even without an alignment film.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2010-33093 A

SUMMARY OF INVENTION Technical Problem

However, the experiments performed by the present inventors haverevealed that some material or production conditions cause a failure inproviding good display even when a liquid crystal composition thatcontains a liquid crystal material, a monomer, a polymerizationinitiator, and the like is injected into between substrates and then apolymer layer is formed by a polymerization reaction under predeterminedconditions.

One monomer for forming the polymer layer is a monomer having an alkylgroup, and examples thereof include lauryl acrylate represented by thefollowing formula (9).

The present inventors have examined and thereby found that the abovelauryl acrylate weakly interacts with liquid crystal molecules, likelycausing alignment defects and reduction in voltage holding ratio (VHR).

Patent Literature 1 proposes to use a symmetric polyfunctional monomerin addition to lauryl acrylate, thereby providing, even without analignment film, an initial voltage holding ratio and a residual DC valueat the same levels as those with an alignment film. Examples of thesymmetric polyfunctional monomer include a symmetric polyfunctionalmonomer represented by the following formula (10):

wherein

X represents acrylate or methacrylate;

A is a structure represented by the following formula (11), (12), or(13);

B is a structure represented by the following formula (14) or (15); and

m and n each are an integer of 0 or 1 or greater.

The examinations performed by the present inventors reveal thefollowing. Specifically, the symmetric polyfunctional monomerrepresented by the formula (10) has no alkyl chain that contributes tovertical alignment. Thus, the symmetric polyfunctional monomer alonecannot form a polymer layer that aligns the liquid crystal moleculesorthogonally to the substrate surface.

The present inventors have further examined the types of a monomer thatcan provide good display without an alignment film. Then, they havefound that a compound represented by the following formula (16) can morestrongly interact with liquid crystal molecules than the lauryl acrylatebecause it has a biphenyl structure, so that the resulting polymer layeralone can provide stable alignment even without an alignment film.

In contrast, the present inventors have also found that, although theuse of the compound represented by the formula (16) can provide stablealignment with a polymer layer alone, this compound reduces the VHR.

The present invention is devised in consideration of the abovesituation, and it aims to provide a liquid-crystal display device whichcan maintain a high voltage holding ratio even without an alignmentfilm.

Solution to Problem

The present inventors have examined various possible causes of VHRreduction, and focused on the fact that the compound of the formula (16)generates radicals by UV irradiation and then polymerize, therebyforming a polymer layer. They have further found the following.Specifically, since the compound of the formula (16) has only onepolymerizable group, the compound may fail to contribute to thepolymerization if this polymerizable group is ionized before formationof a polymer layer. This ionized group floats as impurity ions in theliquid crystal layer, causing VHR reduction.

The present inventors have further examined a monomer that can stabilizethe alignment of liquid crystal molecules and can maintain a highvoltage holding ratio. Thereby, the inventors have developed a novelmonomer represented by the following formula (1) that has a biphenylstructure, which strongly interacts with liquid crystal molecules, analkyl chain, and two polymerizable groups.

As a result, the present inventors have arrived at the solution of theabove problems and completed the present invention.

One aspect of the present invention is a liquid-crystal display devicecomprising

a pair of substrates;

a liquid crystal layer which is sandwiched between the substrates andwhich contains a liquid crystal material; and

a polymer layer which is disposed on a surface of each of the substratesand which controls the alignment of liquid crystal molecules,

the outermost surfaces of both the substrates comprising substantiallyno alignment film,

the polymer layer being formed by polymerizing one or more radicalpolymerizable monomer species added to the liquid crystal layer, and

at least one species of the one or more radical polymerizable monomerspecies being a compound represented by the following formula (1):

wherein

X¹ and X² may be the same as or different from each other, and representacryloyl, methacryloyl, vinyl, or allyl;

a is 0 or 1;

b is 0 or 1;

c is 0 or 1;

R is a C1-C20 alkyl; and

each hydrogen atom may optionally be replaced by halogen, methyl, ethyl,or propyl.

R in the compound of the formula (1) is preferably a C4-C20 alkyl, morepreferably a C8-C20 alkyl, and still more preferably a C16-C20 alkyl.

One of the substrates in the liquid-crystal display device of thepresent invention may be an array substrate and the other may be a colorfilter substrate. The array substrate comprises multiple pixelelectrodes. This makes it possible to control the alignment of liquidcrystal per pixel. The color filter substrate comprises color filters ofmultiple colors disposed at the sites where the color filters overlapthe corresponding pixel electrodes on the array substrate. This makes itpossible to control the displayed color per pixel.

The outermost surfaces of both the substrates comprise substantially noalignment film. The term “alignment film” herein means a monolayer filmor a multilayer film of polyimide, polyamic acid, polyamide,polymaleimide, polysiloxane, polysilsesquioxane, polyphosphazene, or acopolymer thereof, or a film which is formed by oblique deposition ofsilicon oxide. The alignment film can control the alignment of liquidcrystal molecules. In usual liquid-crystal display devices, an alignmentfilm can be formed by directly applying (e.g., application of polyimide)or depositing (e.g., oblique deposition of silicon oxide (SiO)) analignment film material onto the substrate surface constituting thedisplay area. The display area herein means an area showing an image tobe recognized by a viewer and does not include the surrounding area suchas terminal portions. The alignment film is not limited to analignment-treated film as long as a known alignment film material (e.g.,polyimide) is applied onto the film.

Each of the substrates is provided with, on a surface thereof, a polymerlayer that controls the alignment of liquid crystal molecules. Thepolymer layer is formed by polymerizing one or more radicalpolymerizable monomer species added to the liquid crystal layer. Theformation of the polymer layer makes it possible to give initial tiltingin a certain direction to the liquid crystal molecules near the polymerlayer even without an alignment film. For example, when a polymer layeris formed by polymerizing a monomer while the liquid crystal moleculesare aligned with pre-tilt alignment, the resulting polymer layer canhave a structure that aligns the liquid crystal molecules with suchpre-tilt alignment.

Since the monomer represented by the formula (1) has a biphenyl basicstructure, it can be polymerized by light from a light source such asblack light even without a polymerization initiator. Further, since thismonomer has an alkyl chain, the monomer alone can form a polymer layerthat stably aligns the liquid crystal molecules. Additionally, themonomer has two polymerizable groups. Thus, even if one polymerizablegroup is ionized before the formation of a polymer layer, the otherpolymerizable group can contribute to the polymerization. As a result,the monomer does not remain in the liquid crystal layer, so that thepolymer can maintain a high VHR even after UV irradiation.

The liquid-crystal display device of the present invention is notespecially limited by other components as long as it essentiallyincludes the above components.

The proportion of the compound of the formula (1) is preferably notlower than 0.5% by weight and not higher than 2.5% by weight in thewhole liquid crystal composition constituting the liquid crystal layerbefore the polymerization.

At least another one species of the one or more radical polymerizablemonomer species preferably has a cyclic structure and a monofunctionalpolymerizable group.

The radical polymerizable monomer having a cyclic structure and amonofunctional polymerizable group is preferably a compound representedby the following formula (2):

wherein

X represents acrylate, methacrylate, acryloyloxy, methacryloyloxy,vinyl, or allyl;

m is an integer of 0 to 12;

a is 0 or 1;

b is 0 or 1;

R is a C1-C20 alkyl; and

each hydrogen atom may optionally be replaced by halogen, methyl, ethyl,or propyl.

The biphenyl basic structure and the alkyl chain in the compound of theformula (2) are collinear with each other, so that the compound has highlinearity. Combination use of the monomer of the formula (1) and themonomer of the formula (2) can further improve the vertical alignmentperformance of the polymer layer.

R in the compound of the formula (2) is preferably a C4-C12 alkyl, andmore preferably a C8-C12 alkyl. Further, m is preferably an integer of 2or greater, more preferably 4 or greater, and still more preferably 8 orgreater.

The ratio of the radical polymerizable monomer having a cyclic structureand a monofunctional polymerizable group is preferably 10 mol % or moreto the compound of the formula (1) before the polymerization.

The liquid crystal material preferably has negative dielectric constantanisotropy. This enables production of a vertical alignment-modeliquid-crystal display device having a high contrast ratio by forming apolymer layer so as to vertically align the liquid crystal moleculesnear the polymer layer.

Another aspect of the present invention is a process for producing aliquid-crystal display device, comprising the steps of:

injecting a liquid crystal composition that contains a liquid crystalmaterial and one or more radical polymerizable monomer species intobetween a pair of substrates the outermost surfaces of which comprisesubstantially no alignment film; and

applying light to the liquid crystal composition to polymerize theradical polymerizable monomers to thereby form a polymer layer whichcontrols the alignment of liquid crystal molecules on a surface of eachof the substrates,

at least one species of the one or more radical polymerizable monomerspecies being a compound represented by the following formula (1):

wherein

X¹ and X² may be the same as or different from each other, and representacryloyl, methacryloyl, vinyl, or allyl;

a is 0 or 1;

b is 0 or 1;

c is 0 or 1;

R is a C1-C20 alkyl; and

each hydrogen atom may optionally be replaced by halogen, methyl, ethyl,or propyl.

R in the compound of the formula (1) is preferably a C4-C20 alkyl, morepreferably a C8-C20 alkyl, and still more preferably a C16-C20 alkyl.

The process for producing a liquid-crystal display device of the presentinvention comprises the step of injecting a liquid crystal compositionthat contains a liquid crystal material and one or more radicalpolymerizable monomer species into between a pair of substrates theoutermost surfaces of which comprise substantially no alignment film.The liquid crystal material and the radical polymerizable monomers herecan be the same as those described for the liquid-crystal display deviceof the present invention.

The process for producing a liquid-crystal display device of the presentinvention comprises the step of applying light to the liquid crystalcomposition to polymerize the radical polymerizable monomers to therebyform a polymer layer which controls the alignment of liquid crystalmolecules on a surface of each of the substrates. The production processof the present invention enables control of the alignment of liquidcrystal molecules without a known alignment film, and suppressesreduction in the voltage holding ratio. Thus, it can exclude additionalsteps or investment in equipment required for forming an alignment film.

Examples of the step of forming a polymer layer include a step performedwhile a voltage not lower than the threshold value is applied to theliquid crystal layer. When the PSA polymerization is performed byapplying light while applying a voltage not lower than the thresholdvalue to the liquid crystal layer, a polymer can be formed along theliquid crystal molecules in the state of being aligned by the appliedvoltage not lower than the threshold value. Thus, the resulting polymerlayer has a structure that aligns the liquid crystal molecules with apre-tilt angle even after the voltage is removed therefrom.

Examples of the step of forming a polymer layer include a step performedwhile a voltage of not lower than the threshold value is not applied tothe liquid crystal layer. Even without a voltage of not lower than thethreshold value, a polymer layer that controls the alignment of liquidcrystal molecules can be formed. Thus, the operations for applying avoltage can be excluded.

The process for producing a liquid-crystal display device of the presentinvention is not especially limited by other steps as long as itessentially includes the above steps.

Examples of preferable modes of the process for producing aliquid-crystal display device of the present invention include thefollowing modes (a) to (j), which are the same as the preferable modesof the liquid-crystal display device of the present invention.Specifically, the modes are the following:

(a) the mode in which the proportion of the compound of the formula (1)is not lower than 0.5% by weight and not higher than 2.5% by weight inthe whole liquid crystal composition;

(b) the mode in which at least another one species of the one or moreradical polymerizable monomer species has a cyclic structure and amonofunctional polymerizable group;

(c) the mode in which the radical polymerizable monomer having a cyclicstructure and a monofunctional polymerizable group is a compound of theformula (2);

(d) the mode in which R in the compound of the formula (2) is a C4-C12alkyl;

(e) the mode in which R in the compound of the formula (2) is a C8-C12alkyl;

(f) the mode in which m in the compound of the formula (2) is an integerof 2 or greater;

(g) the mode in which m in the compound of the formula (2) is an integerof 4 or greater;

(h) the mode in which m in the compound of the formula (2) is an integerof 8 or greater;

(i) the mode in which the ratio of the radical polymerizable monomerhaving a cyclic structure and a monofunctional polymerizable group is 10mol % or more to the compound of the formula (1); and

(j) the mode in which the liquid crystal material has negativedielectric constant anisotropy.

The process for producing a liquid-crystal display device preferablyfurther comprises a heating step at a temperature not lower than thetemperature 30° C. lower than the phase transition temperature of theliquid crystal material. This step is more preferably performedsimultaneously with the light application step. The heating at atemperature not lower than the temperature 30° C. lower than the phasetransition temperature of the liquid crystal material makes it easy toalign the liquid crystal molecules orthogonally to the substratesurfaces.

The process for producing a liquid-crystal display device preferablyfurther comprises an annealing step (a heat-treating step) before thelight application step. The annealing allows the radical polymerizablemonomers to diffuse inside the liquid crystal layer, making the monomerconcentration in the liquid crystal layer uniform. As a result, thepolymer layer can uniformly be formed on both of the substrates.

Advantageous Effects of Invention

The present invention can provide a liquid-crystal display devicecapable of maintaining a high voltage holding ratio even without analignment film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a liquid-crystaldisplay device of Embodiment 1 before the PSA polymerization.

FIG. 2 is a schematic cross-sectional view showing the liquid-crystaldisplay device of Embodiment 1 after the PSA polymerization.

FIG. 3 is a flow chart for comparing a process for producing aliquid-crystal display device with an alignment film and a process forproducing the liquid-crystal display device of Embodiment 1.

FIG. 4 is a schematic cross-sectional view showing a liquid-crystaldisplay device of Embodiment 2 before the PSA polymerization.

FIG. 5 is a schematic cross-sectional view showing the liquid-crystaldisplay device of Embodiment 2 after the PSA polymerization.

FIG. 6 shows photographs of liquid crystal cells of Example 1 before andafter the UV irradiation.

FIG. 7 shows photographs of liquid crystal cells of Example 2 before andafter the UV irradiation.

FIG. 8 shows the structures of radical polymerizable monomers.

DESCRIPTION OF EMBODIMENTS

The present invention will be described in more detail below referringto the drawings in the following embodiments, but is not limited tothese embodiments.

Embodiment 1

The liquid-crystal display device of the present invention and aliquid-crystal display device produced by the production process of thepresent invention can exert excellent display characteristics when, forexample, they are used as display apparatuses such as televisions,personal computers, mobile phones, and digital signage.

FIG. 1 and FIG. 2 each show a schematic cross-sectional view of theliquid-crystal display device of Embodiment 1. FIG. 1 is before the PSApolymerization and FIG. 2 is after the PSA polymerization. As shown inFIG. 1 and FIG. 2, the liquid-crystal display device of Embodiment 1comprises a pair of substrates consisting of an array substrate 110 anda color filter substrate 120, and a liquid crystal layer 105 sandwichedbetween the array substrate 110 and the color filter substrate 120. Thearray substrate 110 comprises an insulating transparent substrate madeof glass or other material, and conductive lines, pixel electrodes, thinfilm transistors (TFTs), and the like disposed on the transparentsubstrate. The color filter substrate 120 comprises an insulatingtransparent substrate made of glass or other material, and a colorfilter, a black matrix, a common electrode, and the like disposed on thetransparent substrate.

As shown in FIG. 1, the liquid crystal layer 105 contains a liquidcrystal material and one or more radical polymerizable monomer species104 before the PSA polymerization. The liquid crystal materialpreferably has negative dielectric constant anisotropy. The radicalpolymerizable monomers 104 comprise a compound represented by thefollowing formula (1):

wherein

X¹ and X² may be the same as or different from each other and representacryloyl, methacryloyl, vinyl, or allyl;

a is 0 or 1;

b is 0 or 1;

c is 0 or 1;

R is a C1-C20 alkyl; and

each hydrogen atom may optionally be replaced by halogen, methyl, ethyl,or propyl.

R in the compound of the formula (1) is preferably a C4-C20 alkyl, morepreferably a C8-C20 alkyl, and still more preferably a C16-C20 alkyl.

The proportion of the radical polymerizable monomers 104 in the wholeliquid crystal composition constituting the liquid crystal layer beforethe polymerization is preferably not lower than 0.5% by weight and nothigher than 2.5% by weight, and more preferably higher than 1.5% byweight and not higher than 2.0% by weight.

Preferably, annealing (heat treatment) is performed after the liquidcrystal composition containing a liquid crystal material and one or moreradical polymerizable monomer species 104 is injected into between thearray substrate 110 and the color filter substrate 120. The annealingallows the radical polymerizable monomers 104 to diffuse inside theliquid crystal layer 105, making the monomer concentration uniform inthe liquid crystal layer 105. This enables uniform formation of thepolymer layer 107 to be mentioned later on each of the substrates 110and 120.

Thereafter, the liquid crystal layer 105 is irradiated with light whileheated, so that the radical polymerizable monomers 104 generateradicals. These radicals serve as active species, and thus the radicalpolymerizable groups of the radical polymerizable monomers 104 start andsuccessively promote the chain polymerization. As shown in FIG. 2, thepolymer formed by the polymerization precipitates on the substrates 110and 120 to form the polymer layers (PSA layers) 107. The heatingtemperature is preferably a temperature not lower than the temperature30° C. lower than the phase transition temperature of the liquid crystalmaterial. Heating at such a temperature makes it easy to align theliquid crystal molecules orthogonally to the substrate surfaces.

Since the radical polymerizable monomers 104 used in Embodiment 1 eachhave a biphenyl basic structure, they can generate radicals in responseto the light applied from a light source such as a black light to causepolymerization even without a polymerization initiator. A differentlight source may be used for the polymerization. Still, light having ashort wavelength (e.g., wavelength component having a wavelength ofshorter than 330 nm) may deteriorate the panel interior structures,resulting in deterioration in the quality of displays in some cases.Thus, in terms of reliability, a black light (wavelength range: 300 to370 nm) is preferably used.

If a polymerization initiator such as Irgacure 651 is used, matterscleaved by UV irradiation serve as impurities floating in the liquidcrystal, reducing the VHR. Since no polymerization initiator is used inEmbodiment 1, the VHR can be kept high in comparison with the case ofusing a polymerization initiator. Further, since the radicalpolymerizable monomers 104 have polymerizable groups, they areincorporated into the polymer layers 107 during the polymerization.Thus, the amount of the monomers remaining in the liquid crystal layer105 can greatly be reduced.

As shown in FIG. 2, the polymer layer (PSA layer) 107 is formed on asurface of each of the array substrate 110 and the color filtersubstrate 120, each of which has no alignment film, in Embodiment 1. Asealing material 103 is directly attached to the substrates 110 and 120between the array substrate 110 and the color filter substrate 120 alongthe peripheries of the substrates 110 and 120. This sealing material 103seals the liquid crystal layer 105 in between the array substrate 110and the color filter substrate 120. The application of light to theliquid crystal layer 105 is performed after the liquid crystal layer 105is sealed by the sealing material 103. Thus, the polymer layers 107 areformed within the area surrounded by the sealing material 103.

In Embodiment 1, for example, light is applied to the liquid crystallayer 105 while a voltage not lower than the threshold value is appliedthereto in the PSA polymerization. This enables formation of a polymerthat is along the alignment of liquid crystal molecules which arealigned in response to the applied voltage not lower than the thresholdvalue. Thus, the resulting polymer layers give a pre-tilt angle to theliquid crystal molecules even after the voltage is removed. It should benoted that, even when a voltage not lower than the threshold value isnot applied to the liquid crystal layer 105 in the PSA polymerization,polymer layers that induce vertical alignment can be formed when theradical polymerizable monomers in Embodiment 1 are used.

The following will describe the other constituent elements of theliquid-crystal display device of Embodiment 1

In the liquid-crystal display device of Embodiment 1, the arraysubstrate 110, the liquid crystal layer 105, and the color filtersubstrate 120 are stacked in the order set forth from the back side tothe viewing side of the liquid-crystal display device. The back side ofthe array substrate 110 is provided with a polarizing plate. The viewingside of the color filter substrate 120 is also provided with apolarizing plate. These polarizing plates each may further be providedwith a retarder. The polarizing plates may be circularly polarizingplates.

The liquid-crystal display device of Embodiment 1 can be of any types oftransmission, reflective, and transflective. In the case of atransmission or transflective type, the liquid-crystal display device ofEmbodiment 1 further comprises a backlight. The backlight is disposed onthe back side of the array substrate 110 such that the light passesthrough the array substrate 110, the liquid crystal layer 105, and thecolor filter substrate 120 in the order set forth. In the case of areflective or transflective type, the array substrate 110 is providedwith a reflector for reflecting the light from outside. The polarizingplate of the color filter substrate 120 needs to be a circularlypolarizing plate provided with what is called a λ/4 retarder at least ata region utilizing the reflected light for display.

The liquid crystal layer 105 is filled with a liquid crystal materialhaving a characteristic of aligning in a specific direction when acertain voltage is applied. The alignment of the liquid crystalmolecules in the liquid crystal layer 105 is controlled by applicationof a voltage not lower than the threshold value.

The following will describe a process for producing the liquid-crystaldisplay device of Embodiment 1 referring to FIG. 3 for comparing aprocess for producing a liquid-crystal display device with an alignmentfilm and a process for producing the liquid-crystal display device ofEmbodiment 1.

First described is a process for producing a liquid-crystal displaydevice with an alignment film. As shown in FIG. 3, an array substrateand a color filter substrate are washed, and then an alignment filmmaterial is applied to each substrate and subjected to high-temperatureburning (at about 200° C.) After the alignment films are burnt, thealignment films may be rubbed and washed in some cases.

After the alignment films are formed, multiple pillar-shaped photospacers made of an insulating material are formed, or spacers made of aninsulating material are scattered, on one of the substrates. Then, thesubstrates are laminated with each other using a sealing material. Inthe case of the one drop fill method, a liquid crystal material isdropped before the substrates are laminated. In the case of the vacuumfilling method, a liquid crystal material is injected in vacuo after thesubstrates are laminated. The sealing material may be any of thosecurable by heat, those curable by UV irradiation, and those curable byboth of heat and UV irradiation.

The pair of substrates laminated by the above step is cut into a desiredsize. To the surface of each substrate on the side opposite to theliquid crystal layer are attached a polarizing plate, a phase differencefilm, and the like. Further, a gate driver, a source driver, a displaycontrol circuit, and the like are mounted thereon, and a backlight andother components are incorporated thereinto. Thereby, a liquid-crystaldisplay device in accordance with the use is completed.

In contrast, a process for producing the liquid-crystal display deviceof the present invention comprises neither a step of applying analignment film material after washing the array substrate and the colorfilter substrate nor a step of burning the resulting film.Alternatively, the process comprises a step of forming a polymer layerfor controlling the alignment of liquid crystal molecules. Specifically,a liquid crystal composition containing a liquid crystal material andradical polymerizable monomers is injected into between the substratesby the one drop fill method or the vacuum filling method. After theinjection, the liquid crystal composition is preferably annealed(heat-treated). Thereafter, ultraviolet rays are applied to thesubstrates sandwiching the liquid crystal composition, thereby forming apolymer layer for controlling the alignment of liquid crystal molecules.The application of ultraviolet rays is preferably performed duringheating at a temperature not lower than the temperature 30° C. lowerthan the phase transition temperature of the liquid crystal material.

Then, similar to the process for producing a liquid-crystal displaydevice with an alignment film, the laminated substrates are cut into adesired size and a polarizing plate, a phase difference film, and thelike components are attached to the surface of each substrate on theside opposite to the liquid crystal layer. Further, a gate driver, asource driver, a display control circuit, and the like are mountedthereon and a backlight and other components are incorporated thereinto.Thereby, a liquid-crystal display device in accordance with the use iscompleted.

With respect to the liquid-crystal display device of Embodiment 1, thecomposition of the monomers in the polymer layers, the compositionalratio of the monomers in the polymer layers, the residual amounts of themonomers in the liquid crystal layer, and other data can be examined bydisassembling the liquid-crystal display device (e.g., a mobile phone, amonitor, a liquid crystal TV (television), digital signage) andperforming chemical analyses such as nuclear magnetic resonance (NMR),Fourier transform infrared spectroscopy (FT-IR), and mass spectrometry(MS).

Embodiment 2

A liquid-crystal display device of Embodiment 2 is similar to theliquid-crystal display device of Embodiment 1 except that a radicalpolymerizable monomer having a cyclic structure and a monofunctionalpolymerizable group is used in addition to the radical polymerizablemonomers used in Embodiment 1.

FIG. 4 and FIG. 5 each show a schematic cross-sectional view of theliquid-crystal display device of Embodiment 2. FIG. 4 is before the PSApolymerization and FIG. 5 is after the PSA polymerization. As shown inFIG. 4 and FIG. 5, the liquid-crystal display device of Embodiment 2comprises a pair of substrates consisting of an array substrate 210 anda color filter substrate 220, and a liquid crystal layer 205 sandwichedbetween the array substrate 210 and the color filter substrate 220. Thearray substrate 210 comprises an insulating transparent substrate madeof glass or other material, and conductive lines, pixel electrodes,TFTs, and the like disposed on the transparent substrate. The colorfilter substrate 220 comprises an insulating transparent substrate madeof glass or other material, and a color filter, a black matrix, a commonelectrode, and the like disposed on the transparent substrate.

As shown in FIG. 4, the liquid crystal layer 205 contains a liquidcrystal material, a first radical polymerizable monomer 204, and asecond radical polymerizable monomer 206 before the PSA polymerization.The liquid crystal material preferably has negative dielectric constantanisotropy. The first radical polymerizable monomer 204 is the compoundof the formula (1) and the second radical polymerizable monomer 206 is acompound having a cyclic structure and a monofunctional polymerizablegroup.

The proportion of the first radical polymerizable monomer 204 in thewhole liquid crystal composition constituting the liquid crystal layerbefore the polymerization is preferably not lower than 0.5% by weightand not higher than 2.5% by weight, and more preferably not lower than1.0% by weight and not higher than 2.0% by weight. The ratio of thesecond radical polymerizable monomer 206 to the first radicalpolymerizable monomer 204 is preferably 10 mol % or more before thepolymerization.

Preferably, annealing (heat treatment) is performed after the liquidcrystal composition containing a liquid crystal material and the radicalpolymerizable monomers 204 and 206 are sandwiched between the arraysubstrate 210 and the color filter substrate 220. The annealing allowsthe radical polymerizable monomers 204 and 206 to diffuse inside theliquid crystal layer 205, making the monomer concentration uniform inthe liquid crystal layer 205. This enables uniform formation of thepolymer layer 207 to be mentioned later on each of the substrates 210and 220.

Thereafter, the liquid crystal layer 205 is irradiated with light whileheated, so that the radical polymerizable monomers 204 and 206 generateradicals. These radicals serve as active species, and thus the radicalpolymerizable groups of the radical polymerizable monomers 204 and 206start and successively promote the chain polymerization. As shown inFIG. 5, the polymer formed by the polymerization precipitates on thesubstrates 210 and 220 to form polymer layers (PSA layers) 207. Theheating temperature is preferably a temperature not lower than thetemperature 30° C. lower than the phase transition temperature of theliquid crystal material.

As shown in FIG. 5, the polymer layer (PSA layer) 207 is formed on asurface of each of the array substrate 210 and the color filtersubstrate 220, each of which has no alignment film, in Embodiment 2. Asealing material 203 is directly attached to the substrates 210 and 220between the array substrate 210 and the color filter substrate 220 alongthe peripheries of the substrates 210 and 220. This sealing material 203seals the liquid crystal layer 205 in between the array substrate 210and the color filter substrate 220. The application of light to theliquid crystal layer 205 is performed after the liquid crystal layer 205is sealed by the sealing material 203. Thus, the polymer layers 207 areformed within the area surrounded by the sealing material 203.

In Embodiment 2, a second radical polymerizable monomer 206 having acyclic structure and a monofunctional polymerizable group is used inaddition to the first radical polymerizable monomer 204 of the followingformula (1). Examples of the radical polymerizable monomer having acyclic structure and a monofunctional polymerizable group include acompound represented by the following formula (2):

wherein

X represents acrylate, methacrylate, acryloyloxy, methacryloyloxy,vinyl, or allyl;

m is an integer of 0 to 12;

a is 0 or 1;

b is 0 or 1;

R is a C1-C20 alkyl;

each hydrogen atom may optionally be replaced by halogen, methyl, ethyl,or propyl.

In Embodiment 2, the combination use of the compound of the formula (1)and the compound having a cyclic structure and a monofunctionalpolymerizable group enables formation of a polymer layer providingbetter alignment of the liquid crystal molecules.

Example 1

The following gives Example 1 in which a liquid crystal cell of theliquid-crystal display device of Embodiment 1 was actually produced.First, a pair of substrates each having a transparent electrode on thesurface was prepared. The substrates were washed and, without forming analignment film, a sealing material was applied to one substrate andbeads serving as spacers were scattered on the opposed substrate. Then,the substrates were laminated with each other.

Then, a liquid crystal composition containing a liquid crystal materialwith negative dielectric constant anisotropy and the compound of theformula (3) (3-hexadecyl-4,4′-dimethylacryloxybiphenyl) as a radicalpolymerizable monomer was injected into between the substrates.

The liquid crystal composition as a sample A contains 2.0% by weight ofthe compound of the formula (3) in the whole liquid crystal composition.When the compound of the formula (3) is used alone, it precipitates at aconcentration higher than 2.0% by weight in the whole liquid crystalcomposition. Thus, the sample was prepared at a concentration of nothigher than 2.0% by weight. After the liquid crystal composition wasinjected, the liquid crystal cell was heated, in other words, subjectedto the annealing.

Then, unpolarized ultraviolet light (2.57 mW/cm²) was applied to thesubstrates in the normal direction for 30 minutes with no voltageapplication. Simultaneously, the radical polymerizable monomer waspolymerized while the liquid crystal cell was heated at about 90° C. ona hot plate. The source of the unpolarized ultraviolet light was BlackLight FHF-32BLB (Toshiba Lighting & Technology Corp.). The electrodeswere slit-less flat transparent electrodes (ITO). The sample A wasproduced as mentioned above.

The following will show the alignment of liquid crystal molecules andthe result of measuring the voltage holding ratio (VHR) of the sample A.FIG. 6 shows photographs of the liquid crystal cells of Example 1 beforeand after the UV irradiation through polarizing plates in acrossed-Nicols state.

The alignment of the liquid crystal molecules was evaluated by observingthe photographs of the liquid crystal cell before and after the UVirradiation through polarizing plates in a crossed-Nicols state. Whenthe liquid crystal molecules are vertically aligned without alignmentdefects, the liquid crystal cell shows a substantially completely blackscreen.

The voltage holding ratio (VHR) was measured using a LC materialcharacteristics measurement system (Model 6254, TOYO Corp.). The liquidcrystal cell was put into a 70° C. oven and a pulsed voltage (1 V) wasapplied between the electrodes for 60 μs. Then, the electric potentialbetween the electrodes was measured during 16.67 ms of the releaseperiod (the period with no voltage application). In order to observe achange in the VHR in response to the driving of the liquid-crystaldisplay device, an aging test was performed by irradiating the devicewith light from an LED backlight for 1000 hours.

With respect to the alignment of the liquid crystal molecules, theliquid crystal cell as the sample A after the UV irradiation showed asubstantially completely black screen, as shown in FIG. 6. This provesthat the liquid crystal molecules were vertically aligned withoutalignment defects.

The following Table 1 shows the alignment of the liquid crystalmolecules and the result of measuring the initial voltage holding ratio(VHR).

TABLE 1 Sample A Alignment Good Initial VHR (%) 99.1

The results of Table 1 show that the sample A had a VHR of 99.1% afterthe UV irradiation. Even after the aging test, the sample A maintained aVHR as high as 99% or higher. This proves that the resultingliquid-crystal display device was excellent in light resistance and hadhigh long-term reliability. The compound of the formula (3) has twopolymerizable groups. Thus, even if one of the polymerizable groupsgenerates radicals and ionized by ultraviolet irradiation before theformation of a polymer layer, the other polymerizable group cancontribute to the polymerization. As a result, presumably, the compounddoes not remain in the liquid crystal layer and the reduction in VHR issuppressed.

The aforementioned results show that the use of a radical polymerizablemonomer having a biphenyl structure, an alkyl chain, and twopolymerizable groups, as represented by the formula (3), can provide aliquid-crystal display device that is capable of maintaining a highvoltage holding ratio.

Example 2

The following gives Example 2 in which a liquid crystal cell of theliquid-crystal display device of Embodiment 2 was actually produced.

The liquid crystal composition of Example 2 contained a liquid crystalmaterial having negative dielectric constant anisotropy and the compoundof the formula (3) used in Example 1, as well as a compound of thefollowing formula (4) (4-acryloxy-4′-octylbiphenyl). The liquid crystalcell was produced in the same manner as in Example 1 except for thestructure of the liquid crystal composition.

Samples B to D were produced from the liquid crystal compositions havingdifferent concentrations of the compound of the formula (3) and thecompound of the formula (4). The monomer concentrations in the wholeliquid crystal composition of each sample were shown in Table 2. Withrespect to the solubilities of the monomers in liquid crystal, noprecipitation was observed in the samples B to D.

TABLE 2 Sample B Sample C Sample D Concentration of formula (3) 1.6 1.31.0 (wt %) Concentration of formula (4) 0.3 0.5 0.7 (wt %) Formula(4)/Formula (3) (mol %) 20.5 34.6 49.1

The following will show the alignment of liquid crystal molecules andthe result of measuring the voltage holding ratio (VHR) of each of thesamples B to D. FIG. 7 shows photographs of the liquid crystal cells ofExample 2 before and after the UV irradiation through polarizing platesin a crossed-Nicols state. FIG. 8 shows the structures of the radicalpolymerizable monomers.

With respect to the alignment of the liquid crystal molecules, theliquid crystal cells of the samples B to D each showed a substantiallycompletely black screen as shown in FIG. 7. This proves that the liquidcrystal molecules were vertically aligned without alignment defects. Theresults of Examples 1 and 2 show that the compound of the formula (3) incombination with the compound of the formula (4) can form a polymerlayer for controlling the alignment of the liquid crystal molecules atlower concentrations.

The results of Example 2 will be examined below with reference to FIG.8. The compound of the formula (3)(3-hexadecyl-4,4′-dimethylacryloxybiphenyl) alone can provide verticalalignment. However, the monomer has a lower linearity because thebiphenyl basic structure crosses the alkyl chain. In contrast, thecompound of the formula (4) (4-acryloxy-4′-octylbiphenyl) has a higherlinearity because the biphenyl basic structure is in the same line withthe alkyl chain, as shown in FIG. 8. Thus, the combination use of thecompound of the formula (3) and the compound of the formula (4) enablesformation of a polymer layer for controlling the alignment of the liquidcrystal molecules.

The following Table 3 shows the alignment of the liquid crystalmolecules and the results of measuring the initial voltage holding ratio(VHR). The VHR measurement and the aging test were performed under thesame conditions as in Example 1.

TABLE 3 Sample B Sample C Sample D Formula (4)/Formula (3) (mol %) 20.534.6 49.1 Alignment Good Good Good Initial VHR (%) 97.9 97.1 95.6

Based on the results shown in Table 3, the initial VHR was high when theratio of the compound of the formula (4) was 10 mol % or more to thecompound of the formula (3). The lower the ratio of the compound of theformula (4) to the compound of the formula (3) was, the better theinitial VHR was. The VHR after the aging test of each of the samples Bto D showed a tendency similar to that in the case of the initial VHR,and the VHR did not decrease. As a result, the resulting liquid-crystaldisplay device was excellent in light resistance and had high long-termreliability.

The aforementioned results show that the combination use of a radicalpolymerizable monomer having a biphenyl structure, an alkyl chain, andtwo polymerizable groups, as represented by the formula (3), and aradical polymerizable monomer having a cyclic structure and amonofunctional polymerizable group, as represented by the formula (4),can also provide a liquid-crystal display device which is capable ofmaintaining a high voltage holding ratio.

Example 3

The following gives Example 3 in which a liquid crystal cell of theliquid-crystal display device of Embodiment 2 was actually produced.

The liquid crystal compositions of Example 3 contained a liquid crystalmaterial having negative dielectric constant anisotropy and the compoundof the formula (3) used in Example 1, as well as a compound of thefollowing formula (5). They were defined as samples E to G. Thecompounds each represented by the formula (5) in the samples E to G haddifferent values for m in the formula, in other words, different lengthsof the alkyl chain (spacer) between the polymerizable group and thebasic structure. The liquid crystal cell was produced in the same manneras in Example 1 except for the structure of the liquid crystalcomposition.

In the formula, m is an integer of 0 to 12.

The liquid crystal compositions as the samples E to G each contained1.6% by weight of the compound of the formula (3) in the whole liquidcrystal composition and 0.3% by weight of the compound represented byone of the following formulas (6) to (8) in the whole liquid crystalcomposition. Each of the compounds of the formulas (6) to (8) is acompound represented by the formula (5) in which m is 2, 4, or 8.

In Example 3, the alignment and the VHR of the compound of the formula(5) were evaluated with different lengths of the spacer (differentvalues for m). Table 4 shows the alignment of the liquid crystalmolecules and the results of measuring the initial voltage holding ratio(VHR). The measurement of the VHR and the aging test were performedunder the same conditions as in Example 1.

TABLE 4 Sample E Sample F Sample G Length of spacer of formula (5) 2 4 8(number in m) Alignment Good Good Good Initial VHR (%) 97.5 98.0 98.1

The results of Table 4 show that the liquid crystal molecules werevertically aligned to the substrate surfaces after the UV irradiationregardless of the length of the spacer (the value for m) in the compoundof the formula (5). The initial VHR was as high as about 98% in each ofthe samples E to G. The greater the value for m was, the better theresults were. The VHR after the aging test was 98% or higher in each ofthe samples E to G. As a result, the liquid-crystal display device wasexcellent in light resistance and had high long-term reliability.

The aforementioned results show that the combination use of a radicalpolymerizable monomer having a biphenyl structure, an alkyl chain, andtwo polymerizable groups, as represented by the formula (3), and aradical polymerizable monomer having a cyclic structure and amonofunctional polymerizable group, as represented by any of theformulas (6) to (8), can provide a liquid-crystal display device whichis more effectively capable of maintaining a high voltage holding ratio.

REFERENCE SIGNS LIST

-   110, 210: array substrate (provided with a transparent electrode)-   120, 220: color filter substrate (provided with a transparent    electrode)-   103, 203: sealing material-   104, 204: first radical polymerizable monomer-   105, 205: liquid crystal layer-   107, 207: polymer layer (PSA layer)-   206: second radical polymerizable monomer

1: A liquid-crystal display device comprising a pair of substrates; aliquid crystal layer which is sandwiched between the substrates andwhich contains a liquid crystal material; and a polymer layer which isdisposed on a surface of each of the substrates and which controls thealignment of liquid crystal molecules, the outermost surfaces of boththe substrates comprising substantially no alignment film, the polymerlayer being formed by polymerizing one or more radical polymerizablemonomer species added to the liquid crystal layer, and at least onespecies of the one or more radical polymerizable monomer species being acompound represented by the following formula (1):

wherein X¹ and X² may be the same as or different from each other, andrepresent acryloyl, methacryloyl, vinyl, or allyl; a is 0 or 1; b is 0or 1; c is 0 or 1; R is a C1-C20 alkyl; and each hydrogen atom mayoptionally be replaced by halogen, methyl, ethyl, or propyl. 2: Theliquid-crystal display device according to claim 1, wherein R in thecompound of the formula (1) is a C4-C20 alkyl. 3: The liquid-crystaldisplay device according to claim 1, wherein R in the compound of theformula (1) is a C8-C20 alkyl. 4: The liquid-crystal display deviceaccording to claim 1, wherein R in the compound of the formula (1) is aC16-C20 alkyl. 5: The liquid-crystal display device according to claim1, wherein at least another one species of the one or more radicalpolymerizable monomer species has a cyclic structure and amonofunctional polymerizable group. 6: The liquid-crystal display deviceaccording to claim 5, wherein the radical polymerizable monomer having acyclic structure and a monofunctional polymerizable group is a compoundrepresented by the following formula (2):

wherein X represents acrylate, methacrylate, acryloyloxy,methacryloyloxy, vinyl, or allyl; m is an integer of 0 to 12; a is 0 or1; b is 0 or 1; R is a C1-C20 alkyl; and each hydrogen atom mayoptionally be replaced by halogen, methyl, ethyl, or propyl. 7: Theliquid-crystal display device according to claim 6, wherein R in thecompound of the formula (2) is a C4-C12 alkyl. 8: The liquid-crystaldisplay device according to claim 6, wherein R in the compound of theformula (2) is a C8-C12 alkyl. 9: The liquid-crystal display deviceaccording to claim 6, wherein m in the compound of the formula (2) is aninteger of 2 or greater. 10: The liquid-crystal display device accordingto claim 6, wherein m in the compound of the formula (2) is an integerof 4 or greater. 11: The liquid-crystal display device according toclaim 6, wherein m in the compound of the formula (2) is an integer of 8or greater. 12: The liquid-crystal display device according to claim 1,wherein the proportion of the compound of the formula (1) is not lowerthan 0.5% by weight and not higher than 2.5% by weight in the wholeliquid crystal composition constituting the liquid crystal layer beforethe polymerization. 13: The liquid-crystal display device according toclaim 5, wherein the ratio of the radical polymerizable monomer having acyclic structure and a monofunctional polymerizable group is 10 mol % ormore to the compound of the formula (1) before the polymerization. 14:The liquid-crystal display device according to claim 1, wherein theliquid crystal material has negative dielectric constant anisotropy. 15:A process for producing a liquid-crystal display device, comprising thesteps of: injecting a liquid crystal composition that contains a liquidcrystal material and one or more radical polymerizable monomer speciesinto between a pair of substrates the outermost surfaces of whichcomprise substantially no alignment film; and applying light to theliquid crystal composition to polymerize the radical polymerizablemonomers to thereby form a polymer layer which controls the alignment ofliquid crystal molecules on a surface of each of the substrates, atleast one species of the one or more radical polymerizable monomerspecies being a compound represented by the following formula (1):

wherein X¹ and X² may be the same as or different from each other, andrepresent acryloyl, methacryloyl, vinyl, or allyl; a is 0 or 1; b is 0or 1; c is 0 or 1; R is a C1-C20 alkyl; and each hydrogen atom mayoptionally be replaced by halogen, methyl, ethyl, or propyl. 16-28.(canceled) 29: The process for producing a liquid-crystal display deviceaccording to claim 15, further comprising a heating step at atemperature not lower than the temperature 30° C. lower than the phasetransition temperature of the liquid crystal material. 30: The processfor producing a liquid-crystal display device according to claim 15,further comprising: an annealing step before the light application step.